Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace which has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products including olefins leave the pyrolysis furnace for further downstream processing, including quenching.
Pyrolysis involves heating the feedstock sufficiently to cause thermal decomposition of the larger molecules. The pyrolysis process, however, produces molecules which tend to combine to form high molecular weight materials known as tar. Tar is a high-boiling point, viscous, reactive material that can foul equipment under certain conditions. In general, feedstocks containing higher boiling materials tend to produce greater quantities of tar.
The formation of tar after the pyrolysis effluent leaves the steam cracking furnace can be minimized by rapidly reducing the temperature of the effluent exiting the pyrolysis unit to a level at which the tar-forming reactions are greatly slowed. This cooling which may be achieved in one or more steps and using one or more methods is referred to as quenching.
Conventional steam cracking systems have been effective for cracking high-quality feedstock which contain a large fraction of light volatile hydrocarbons, such as gas oil and naphtha. However, steam cracking economics sometimes favor cracking lower cost heavy feedstocks such as, by way of non-limiting examples, crude oil and atmospheric residue. Crude oil and atmospheric residue often contain high molecular weight, non-volatile components with boiling points in excess of 1100° F. (590° C.) otherwise known as resids. The non-volatile components of these feedstocks lay down as coke in the convection section of conventional pyrolysis furnaces. Only very low levels of non-volatile components can be tolerated in the convection section downstream of the point where the lighter components have fully vaporized.
In most commercial naphtha and gas oil crackers, cooling of the effluent from the cracking furnace is normally achieved using a system of transfer line heat exchangers, a primary fractionator, and a water quench tower or indirect condenser. The steam generated in transfer line exchangers can be used to drive large steam turbines which power the major compressors used elsewhere in the ethylene production unit. To obtain high energy-efficiency and power production in the steam turbines, it is necessary to superheat the steam produced in the transfer line exchangers.
Cracking heavier feeds, such as kerosenes and gas oils, produces large amounts of tar, which lead to rapid coking in the radiant section of the furnace as well as fouling in the transfer line exchangers preferred in lighter liquid cracking service.
Additionally, during transport some naphthas are contaminated with heavy crude oil containing non-volatile components. Conventional pyrolysis furnaces do not have the flexibility to process residues, crudes, or many residue or crude contaminated gas oils or naphthas which are contaminated with non-volatile components.
To address coking problems, U.S. Pat. No. 3,617,493, which is incorporated herein by reference, discloses the use of an external vaporization drum for the crude oil feed and discloses the use of a first flash to remove naphtha as vapor and a second flash to remove vapors with a boiling point between 450 and 1100° F. (230 and 590° C.). The vapors are cracked in the pyrolysis furnace into olefins and the separated liquids from the two flash tanks are removed, stripped with steam, and used as fuel.
U.S. Pat. No. 3,718,709, which is incorporated herein by reference, discloses a process to minimize coke deposition. It describes preheating of heavy feedstock inside or outside a pyrolysis furnace to vaporize about 50% of the heavy feedstock with superheated steam and the removal of the residual, separated liquid. The vaporized hydrocarbons, which contain mostly light volatile hydrocarbons, are subjected to cracking. Periodic regeneration above pyrolysis temperature is effected with air and steam.
U.S. Pat. No. 5,190,634, which is incorporated herein by reference, discloses a process for inhibiting coke formation in a furnace by preheating the feedstock in the presence of a small, critical amount of hydrogen in the convection section. The presence of hydrogen in the convection section inhibits the polymerization reaction of the hydrocarbons thereby inhibiting coke formation.
U.S. Pat. No. 5,580,443, which is incorporated herein by reference, discloses a process wherein the feedstock is first preheated and then withdrawn from a preheater in the convection section of the pyrolysis furnace. This preheated feedstock is then mixed with a predetermined amount of steam (the dilution steam) and is then introduced into a gas-liquid separator to separate and remove a required proportion of the non-volatiles as liquid from the separator. The separated vapor from the gas-liquid separator is returned to the pyrolysis furnace for heating and cracking.
Co-pending U.S. application Ser. No. 10/188,461 filed Jul. 3, 2002, Patent Application Publication US 2004/0004022 A1, published Jan. 8, 2004, which is incorporated herein by reference, describes an advantageously controlled process to optimize the cracking of volatile hydrocarbons contained in the heavy hydrocarbon feedstocks and to reduce and avoid coking problems. It provides a method to maintain a relatively constant ratio of vapor to liquid leaving the flash by maintaining a relatively constant temperature of the stream entering the flash. More specifically, the constant temperature of the flash stream is maintained by automatically adjusting the amount of a fluid stream mixed with the heavy hydrocarbon feedstock prior to the flash. The fluid can be water.
U.S. patent application Ser. No. 60/555,282, filed Mar. 22, 2004, which is incorporated herein by reference, describes a process for cracking heavy hydrocarbon feedstock which mixes heavy hydrocarbon feedstock with a fluid, e.g., hydrocarbon or water, to form a mixture stream which is flashed to form a vapor phase and a liquid phase, the vapor phase being subsequently cracked to provide olefins, and the product effluent cooled in a transfer line exchanger, wherein the amount of fluid mixed with the feedstock is varied in accordance with a selected operating parameter of the process, e.g., temperature of the mixture stream before the mixture stream is flashed.
In using a flash to separate heavy liquid hydrocarbon fractions from the lighter fractions which can be processed in the pyrolysis furnace, it is important to effect the separation so that most of the non-volatile components will be in the liquid phase. Otherwise, heavy, coke-forming non-volatile components in the vapor are carried into the furnace causing coking problems. However, the flashing in a flash/separation vessel is typically accompanied by coking of internal surfaces in and proximally downstream of the vessel. The extent of such coking is dependent upon various factors including feed type, preheating protocol, and design of the vessel. Liquids contacting the internal surfaces of the vessel and downstream equipment provide coatings of films that are precursors to coke. Excessive temperatures, such as above about 427° C. (800° F.), typically from about 450 to about 460° C. (840 to about 860° F.) or from about 510° to above about 621° C. (950° F. to 1150° F.), depending on the feedstock, are theorized to lead to excessive coke formation by thermal cracking and heat soaking of the heavy end of the heavy hydrocarbon feedstock stream. Because this coke buildup can effect restriction and increase pressure drop within the overall process, it would be advantageous to control the buildup within the flash zone and immediately downstream of the flash zone.